Definition for Asthenosphere
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The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~62 and 124 miles) below the surface, but perhaps extending as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt.